I am a prospective homebuyer and found a house I really liked. I was as far as had an agreed price and signed contract, but during the home inspection found what appeared to be mold in the attic (along with a few other minor issues). Being that the house was already at the high end of my price range, I ended up killing the contract.

A few months have passed and the home owner hired a contractor to address and fix everything that came up during my home inspection. He examined the discoloration in the attic and claims that it is not mold.

The contractor says that the discoloration is the result of frost occasionally appearing in the attic and melting. He claims that the attic area is meant to be as cold as the outdoors, as the insulation is in the attic floor rafters. He also said its not unusual for frost to form on the underside of the plywood under certain conditions.

Below are a couple of pictures I have from my home inspection. They are small size and not so great quality, so I apologize in advance but its all I have.

I realize its probably very difficult to tell, but do you think there could be any truth to what this contractor is saying? He has his own business and has allegedly been doing new home construction and remodeling for a very long time. My thoughts at this point are to either let the deal stay dead, or hire/negotiate to have some mold professionals check the spots. Additionally, if I were to go the latter route, would it be worth having an indoor air quality test done?

I appreciate any thoughts! I know I'm certainly not the most well versed person in regards to these kind of things, but if there's any information I can give that would help let me know.

Welcome to the site.. He may be right and it might not be mold and it dosn't look to bad if it is, fixable. But if he is right about ice forming on the inside and that is a problem and that detail may have been left out of what the contracter reported. The problem there is warm moist air in the attic. The venting system in the attic should let warm air out the vents on top of the roof and replace the air with cool dry air from vents under the eaves. So you might have warm moist air leaking from the living space below, not enough insulation to keep the attic cool or not enough soffet vent (eaves) or insulation plugging the bays between the rafters over the exterior walls not allowing air to flow up the roof. If this is the only problem, it looks early in the process but I would have it inspected by someone looking at these details and he should take a sample from the questionable spot for testing and then have it tested at a lab.

Not sure how relevant this is, but the woman who was living in the house before was frequently using an unvented standalone propane fireplace in the living room. Is it possible that this would cause additional condensation?

The house is two stories, but a sort of A-frame style. The living room is on the first floor which is open to the ceiling below the attic.

There should be no air leakage from inside the house, so maybe is best guess. I would expect hints of that to show on the inside but may have been cleaned up.
If you were their best prospective buyer, I would lower the price a little too, just because you can.

Look carefully. When you first saw the problem, you should have done a tape lift to trap whatever was on the underside. That sealed tape can be sent for a test quite easily. Alternatively, a home inspection and lab test should cost less than $240.

What was on your roof? Black toxic mold usually looks black or dark green. When a black toxic mold is growing it has a distinctive shiny, slimy looking surface. However if black toxic mold dries out it can turn gray and powdery looking.

Toxic mold is usually only found in houses with significant water damage, usually caused by flooding or persistent leaks which weren't repaired for a long time. Once toxic mold starts to grow it still needs constant moisture to keep on growing. What makes black toxic mold harmful is that it releases mycotoxins. These are immunosuppressant and can cause a variety of symptoms in adults and children.

I have this on my roof sheathing too. Took to long on my remodel I guess letting warm air into attic like neal said. I sprayed some industrial mold killer on it and sealed the the attic from house better. The black is still there wood is stained I hope that it is dead but since air does not travel between attic and living space I am not concerned health wise. I wouldn't let it kill the deal
check that none of bathroom vents , vent directly into attic.

ive seen a great tool called a cold fogger. you can use it in conjunction with a mold killer. i dont think you have to buy a pro model. they sell less expensive cold foggers. http://www.concrobium.com/products/c...ontrol-fogger/
anyways it might help treat it without ripping it all out....
it may not even be needed but i thought i throw it out there...

The staining could be from many sources. The one that seems most likely is the sheathing having been left out in the weather during construction. I've seen plenty of mold and what you have doesn't look much like it.
FWIW
YMMV